In Vim, you can type :%!xxd to turn it into a hex editor. :%!xxd -r to go back to normal mode. xxd is shipped in a Vim installation.
See here for some remarks about editing binary files with Vim (it boils down to :set binary to avoid trouble. Use only the “R” or “r” command to change text, and don’t delete characters).
If you are an Emacs fan, see here for a guide on how to edit a binary file with Emacs.